Major Heat Risk for portions of Pima, southeastern Pinal, and southern Graham and Greenlee counties Saturday and Sunday. Heat is the #1 cause of weather-related fatalities in the United States. Prolonged exposure to heat can result in heat stroke and heat exhaustion. Increase risk for heat-related illnesses for pets, livestock, and much of the population, especially with extended outdoor exposure, and for those who are heat sensitive and/or without effective cooling and/or adequate hydration.